The present invention relates to a grinding unit comprising at least                a stationary ring burr with an opening for receiving a rotary frustoconical burr to delimit a grinding gap there between, which grinding gap has an outlet for grinded matter at a base of the frustoconical burr and an inlet for matter to be grinded at the opposite top end face of the frustoconical burr,        the ring burr has an interior wall provided with annularly spaced apart first curved teeth protruding radially into the grinding gap, and        the frustoconical burr has an annular exterior wall provided with annularly spaced apart second curved teeth protruding into the grinding gap.        
Grinders for grinding various kinds of solid matter, such as coffee and spices are commonly known, and depending on the nature of the granular matter to be downsized by grinding and the end purpose of the grinded matter, the grind size is particular relevant. For example coffee drinkers require different grinds for different purposes and grinds size and grinding operation influences preservation of flavour and aroma of the grinded granular matter. Coffee drinkers may for example require coarsely ground coffee beans for press pots, and more finely ground coffee beans for drip filter machines and espresso, and even finer ground coffee for Turkish coffee. How coarse or fine grind or milling size, which is desired, often comes down to individual taste. It should be noted that most coffee grinders cannot produce the very fine grinding grade needed for Turkish coffee.
There is a demand for alternative devices for grinding granular or particulate matter or re-grinding particulate matter further or a second time in a manner that sets free any content of aromatic compounds, e.g. so that such aromatic compounds can be extracted, as is the case when brewing coffee.
European patent application EP 1818099A1 and EP 1818100A1 both disclose grinding devices including a frustoconical burr mill arranged for grinding particulate and granular matter such as corn or particles of e.g. pepper, corn, salt, sugar and coffee. These known grinding devices comprise a milling cone, the cone burr, and a surrounding milling ring, the ring burr, such that a grinding gap is formed between the cone and the ring. The cone and surrounding ring may e.g. be manufactured in metal or ceramics. The cone comprises five teeth running along the outer surface of the cone, from the top to the bottom of the cone. These teeth on the cone serve to move and transport the corn or particles. The ring on the other hand is presented as having several grooves running along it's inner surface, from the top to the bottom of the ring.
German patent application DE19514794A1 discloses a similarly structured grinding unit also having a frustoconical burr, a truncated cone, enclosed in a fixed burr ring leaving a grinding gap between them. The burr ring and frustoconical burr each have six opposite teeth and grinding grooves facing against each other. The teeth slope at acute angles to the vertical direction of travel and the teeth decrease in height towards the maximum cone diameter.
The inventor of the present invention has realised that if solid food granules and/or particles having sizes of e.g. between 2-10 mm are grinded the grinding path from inlet to outlet is too short to provide a resulting grinded product complying with food enthusiasts demand for high quality properties, e.g. properties relating to extracting aromatic compounds, and to dispersion properties. The grinding operation of known grinders requires many turns and thus long time, moreover inlet gap of known grinding units is rather large so that many turns are required to operate the grinding unit for goods grinding results. Raw matter and grinded matter often have very inhomogeneous particle sizes, and manual grinding requires extra effort.
A further major disadvantage of known frustoconical burr mills is that they cannot re-grind for example coffee. Ground coffee cannot be made finer because the burrs rapidly clog. Only whole beans or very course granules can be grinded and manufacturers of coffee grinders with ring burr and cone burr instruct and warn users accordingly in accompanying instruction manuals. Even if instruction manuals are followed maintenance must be made frequently to prevent clogging.
The increasing demand for effective, fast grinding of e.g. coffee beans, without loosing taste and aroma, put focus on grinding devices and requires improvement of such known devices.